1. Field of the Invention
This invention relates to injection molding and in particular to injection molding wherein the molded element is formed by delivery of fluid synthetic resin to a cavity through a plurality of gates.
2. Description of the Background Art
A problem arises in the molding of a synthetic resin element from deliveries of the fluid resin through different gates to the mold cavity. More specifically, the joint formed by the leading surfaces of the several deliveries of fluid resin received through the different gates when they meet in the mold cavity is well known to be a potentially weak portion of the element. A number of attempted solutions to this problem have been made in the prior art while none has proven completely satisfactory.
More specifically, James S. Reid shows, in U.S. Pat. No. 2,182,389, a zone wherein the different streams of the resin meet in the cavity which is heated to render the leading edges of the streams more plastic at that zone.
Warren R. Tucker discloses, in U.S. Pat. No. 2,360,023, a similar structural concept.
Elmer L. Danielson et al., in U.S. Pat. No. 2,607,937, teaches a method of molding different parts sequentially and is not concerned with the problem of a heat weld joint.
A method of forming an annular plastic article by firstly filling a channel and then substantially simultaneously delivering the plastic from the channel into the annular mold is disclosed in U.S. Pat. No. 2,878,517 of Trevor K. Cramer.
In U.S. Pat. No. 3,051,994, Giorgio Carozzo shows an injection molded article wherein means are provided for causing a sharp demarcation line between two plastic sections of the molded article.
Another co-injection molding apparatus is illustrated in U.S. Pat. No. 3,061,879 of Ramon R. Montpeat.
Theodore G. Apostoleris et al., in U.S. Pat. No. 3,880,978, show a method for forming a plastic article wherein the mold design is arranged to prevent radial alignment of the welds from a single sprue.
I. Martin Spier shows, in U.S. Pat. No. 3,950,483, an injection molding process wherein dissimilar fluids intermixed at transitional zones provide the desired one-piece construction. The desired qualities are obtained by selecting dissimilar starting materials and molding an integrally cured and set article therefrom.
David O. Allen shows an injection molding method in U.S. Pat. No. 4,128,613 utilizing a gate having an irregular shape to avoid drooling, stringing, freeze-off or blockage problems.
Dwight M. Anderson shows a method and means for eliminating the weld line during thermoplastic molding in U.S. Pat. No. 2,191,703 by providing mold relief pockets adjacent the mold cavity portion wherein the weld line would otherwise form. Anderson teaches that by providing a choked or restricted portion of the mold cavity adjacent an overflow pocket, an improved surface on the article can be obtained.